1. Field of the Invention
The present invention relates generally to the field of corrosion-inhibiting pigments and is particularly concerned with such pigments based on chromium compounds.
2. Prior Art
Metal protective pigments have long been used as components of coating compositions applied to metallic surfaces. Certain of these pigments have been, at least partially, effective in providing a specific inhibiting or passivating action at the surface of the metal. Among the useful pigments in this category are red lead and certain chromium containing pigments.
Red lead has been used for years in large quantities in the formulation of metal protective coatings having good rust-inhibitive properties. Such inhibitive activity is attributed, in part, to the fact that red lead is basic in reaction and further, to the formation of a protective metal oxide layer on the treated metal surface, which layer resists attack by corrosive elements.
Among the better known chromium-containing pigments suggested for use in metal protecting coatings are zinc yellow or so called "zinc chromate" and zinc tetroxy chromate. These, among other pigments, are described in Kirk-Othmer: Encyclopedia of Chemical Technology, 2d ed. Vol. 5, at pages 507-510; and by V. H. Chalupski in the monograph appearing in UDY: Chemistry of Chromium and its Compounds, Vol. 1, (Rheinhold Pub. Co. 1956) beginning at page 357, particularly pages 375-381. A further modified method for preparing zinc yellow is described in U.S. Pat. No. 2,410,916.
Among other chromium pigments asserted to possess corrosion inhibitive properties are included chromates of alkaline earth metals and respectively of lead and iron, which pigments may also include alkali metal cation (potassium). A number of these have been commercially exploited to some degree.
U.S. Pat. No. 2,387,528 describes alkaline earth metal chromates containing trivalent as well as hexavalent chromium, such as basic calcium chromito-chromate. These pigments, because of various drawbacks, have not achieved commercial significance; UDY, op. cit., at page 381; Kirk-Othmer, op. cit., Vol. 5 at page 510. These are prepared, according to the patent, by roasting a mixture of the alkaline earth chromate and an alkaline earth metal compound which is heat decomposable to the oxide, in designated proportions and grinding the resulting product.
In U.S. Pat. No. 2,430,589 metal protective pigments are described, composed of calcium chromate intimately associated with a minor portion of an admixed ferric, manganic or chromic oxide.
Chromic acid and its salts are included among the most effective corrosion inhibitive substances. The soluble chromates are recommended as among the best materials for use in protecting corrosion of metals such as iron, steel, zinc, aluminum copper, and alloys of these. The exact mechanism of corrosion inhibition is not known with certainty, but it is widely believed that protection is effected by the action of chromate ions made available by controlled solution on the metal surface. In the presence of chromates, it is generally thought, the local anodes that serve as corrosion centers become highly polarized. Thus, in the presence of soluble chromate, the metal appears to be more "noble".
Chromic acid and other soluble chromium-containing compounds have accordingly been used in aqueous metal treating or rinsing solutions applied to the metal surface or to the "conversion" coating thereon, to improve corrosion resistance and paint bonding characteristics. Thus, U.S. Pat. No. 2,902,394 describes protective metal finishing compositions for application to zinc surfaced articles, prepared by intimate admixture of chromic acid with aqueous nitric, sulfuric and acetic acids and with a sulfite-type reducing agent capable of reducing a minor portion of the hexavalent chromium to trivalent state.
An earlier patent of the present inventor, U.S. Pat. No. 3,063,877, describes aqueous solutions for treating metal surfaces to impart improved corrosion resistance, which solutions are prepared by partially reducing a dissolved hexavalent chromium compound with formaldehyde, under conditions such that at least 5% and preferably at least 20% of the total chromium is present in reduced state.
U.S. Pat. No. 3,279,958 describes rinsing of phosphate, chromate and other chemical conversion coatings on metal surfaces with a dilute aqueous acidic solution of a chromium-chromate complex, followed by a water rinse. The complex is prepared by treating aqueous chromic acid solution with an organic reducing agent which contains an active hydroxyl, aldehyde or carboxyl group, to reduce a portion of the hexavalent chromium to the trivalent state.